1. Field of the Invention
The present invention relates to a thermal-type airflow meter suitable for measuring the flow rate of air taken into an internal combustion engine, and more particularly to a thermal-type airflow meter, which comprises a housing for protectingly accommodating an electronic circuit therein and a. sub-air passage.
2. Description of the Related Art
JP-A 3-233168 discloses an example of prior art thermal-type airflow meters. In this prior art, a housing part for protectingly accommodating an electronic circuit therein and a sub-air passage part are arranged in the same plane to be made one body. Further, there is also proposed an airflow meter, in which a housing and a sub-air passage are plastic-molded as a unitary body.
However, the prior art as mentioned above was necessary to be further improved in the following points:
(1) A member, in which a housing and a sub-air passage are formed, has the shape suitable for assembling by means of insertion technology (to be proper in the dimension of an inserting hole and the size of a member to be inserted);
(2) The positional relationship of a housing and a sub-air passage never varies. That is, after the sub-air passage was fixedly attached to a member, in which a main air passage is formed, the attaching position of the subair passage never varies;
(3) The deformation of an airflow meter due to the environmental condition of usage is as small as possible, and the durability and reliability thereof is to be sufficient; and
(4) There is the freedom in designing the shape of a sub-air passage, and further even if it has the complicated shape, it can be easily manufactured.
An object of the present invention is to provide a thermal-type airflow meter which can measure the flow rate of air at the high accuracy and has the excellent durability and reliability against the environment of usage. Further, the present invention provides an intake air system for an internal combustion engine and a control system therefor, which is high in its control ability and reliability.
A thermal-type airflow meter according to a feature of the present invention comprises a sub-air passage which is installed therein with a flow rate measuring part including a heating resistor and a housing, arranged next to the sub-air passage, which accommodates therein an electronic control part for controlling the heating of the flow rate measuring part, wherein the sub-air passage and the housing are plastic-molded separately, then fixedly attached to each other and put on a flat metallic base member with the mechanical strength in its longitudinal direction, whereby the sub-air passage and the housing are fixed on the metallic base member.
Preferably, in the adhesion among the sub-air passage, the housing and the metallic base member, a face, on which different two of the aforesaid members come into contact with each other, is commonly owned by the two members.
According to another feature of the present invention, a thermal-type airflow meter has a sub-air passage having at least one bent portion, through which a part of air in a main air passage as an intake air passage of an internal combustion engine flows, a heating resistor arranged within the sub-air passage and an electronic circuit, electrically connected to the heating resistor, for producing a signal according to an air flow rate on the basis of heat radiated by the heating resistor, wherein a housing protectingly accommodating therein the electronic circuit is composed of a frame part surrounding the electronic circuit, a connector part with connector terminals for electrically connecting the electronic circuit with external devices, and fixing mounts for fixedly supporting the heating resistor on the opposite side to the connector part with respect to the frame part, all of which being plastic-molded as one body, in which a member for forming the sub-air passage is a plastic-molded article and at least the frame part of the housing and the sub-air passage member are formed in a flat shape and fixedly arranged on a flat metallic base member in the longitudinal direction thereof so as to locate the heating resistor in the sub-air passage.
According further to another feature of the present invention, a housing protectingly accommodating therein the electronic circuit comprises a frame part surrounding the electronic circuit, a connector part with connector terminals for electrically connecting the electronic circuit with external devices, and fixing mounts for fixedly supporting the heating resistor on the opposite side to the connector part with respect to the frame part, all of which being plastic-molded as one body, in which a member for forming the sub-air passage is a plastic-molded article and at least the frame part of the housing and the sub-air passage member are formed in a flat shape and fixedly arranged on a flat metallic base member in the longitudinal direction thereof, and there is provided a cover for covering an opposite side of the metallic base member to the frame part, which cover is fixed both to the housing and the sub-air passage member, whereby the frame part is covered by the metallic base member on one side and by the cover on the opposite side to thereby form a closed room for accommodating therein the electronic circuit.
The thermal-type airflow meter according to the present invention can be attached to a part of a body of an air cleaner. Further, an intake air system for a combustion engine performs the fuel control using such airflow meter.
According to the present invention, since a housing and a member of a sub-air passage are separately plastic-molded, the dimension of both in the longitudinal direction can be shortened. As a result, the initial dimensional change of those members during molding can be suppressed, and the housing and the sub-air passage member are adhered to each other and both are fixedly arranged on a metallic base member with the mechanical strength to suppress the dimensional change upon assembling and the aged one.
With these, the dimensional accuracy can be secured and the influence of the dimensional change on the measuring accuracy can be reduced, whereby the accurate measurement of the air flow rate can be achieved. Further, one member is adhered to more than one member which is different from each other. Accordingly, the frequency of separation or removal of two members is remarkably reduced, with the result that the durability and reliability of the airflow meter is much improved.